Method for tungsten nucleation from WF6 using titanium as a reducing agent

ABSTRACT

A method in a semiconductor process for forming a layer of a selected compound on a substrate of a semiconductor device. A layer of titanium is formed on the substrate as a sacrificial layer. The layer of titanium is reduced using a gaseous form of a fluorine containing compound in which the fluorine containing compound includes the selected compound that is to be formed on the substrate of the semiconductor device.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to semiconductor devices and inparticular to a process for manufacturing semiconductor devices. Stillmore particularly, the present invention relates to an improved methodfor forming tungsten plugs.

2. Description of the Related Art

The fabrication of semiconductor devices include processes thatfrequently require the deposition of thin layers onto the semiconductorsurface. This deposition can be achieved by sputtering, but anotherimportant process for deposition of materials is chemical vapordeposition (CVD), which is regularly used for silicon dioxide, siliconnitride, and polycrystalline silicon (polysilicon). Basically, amaterial is reacted at a high temperature and is deposited on thesurface of the semiconductor with the aid of a carrier gas. Vapor phaseepitaxial techniques are a good example of this process. Depositionoften takes place at low pressures. Improved film quality, however, canbe obtained by allowing the reaction to take place at low pressures inwhat is known as a low pressure CVD (LPCVD) reactor. Chemical vapordeposition is not confined to polysilicon, silicon oxide, and siliconnitride. Other materials such as tungsten and tungsten silicide (WSi₂)among others have been deposited using chemical vapor deposition.

Tungsten chemical vapor deposition (CVD) plugs are typically depositedby the reduction of WF₆ by gaseous reducing agents such as silane (SiH₄)and hydrogen gas (H₂) are commonly used for vertically connectingadjacent layers of aluminum in sub 1.0 μm applications. Tungsten istypically deposited on a titanium nitride (TiN)/titanium (Ti) liner inwhich the TiN layer acts as a diffusion barrier to prevent WF₆ fromreacting with the underlying layers. Titanium getters the interfacialimpurities such as oxygen from the surface of the underlayers such asaluminum and silicon and reduces contact and via resistance. On othersubstrates such as silicon or silicon oxide, titanium also serves as anadhesion promotor for overlayers by forming a thin TiSi₂ on silicon andTiO_(x) Si_(y) on silicon oxide layer.

Delamination of sputter-deposited TiN/Ti liners during the early stagesof tungsten CVD is a major defect issue during semiconductor deviceprocessing. The delamination causes the formation of volcano-likedefects that cause metal bridging or open metal lines, which often leadsto device failure. WF₆ diffuses through intercolumnar voids in the TiNlayer and reacts with the underlying Ti layer during tungsten CVD. Theresulting accumulation of large quantities of fluorine in the titaniumunderlayer interacts with the Ti/SiO₂ interface, causing adhesionfailure. Basically, fluorine diffuses through the bilayer of TiN and/Tiduring the reduction of WF₆ by SiH₄ and ₂ H . The nucleation of tungstenusing SiH₄, and H₂ or both is too slow to provide adequate protectionfrom fluorine diffusion, depending on the structure of the TiN film.

Therefore, it would be advantageous to have an improved method fordepositing tungsten on a semiconductor device.

SUMMARY OF THE INVENTION

The present invention provides a method in a semiconductor process forforming a layer of a selected compound on a substrate of a semiconductordevice. A layer of titanium is formed on the substrate as a sacrificiallayer. The layer of titanium is reduced using a gaseous form of afluorine containing compound in which the fluorine containing compoundincludes the selected compound that is to be formed on the substrate ofthe semiconductor device.

An example of a fluorine containing compound is WF₆, which reduces atitanium layer to form a layer of tungsten on the substrate of thesemiconductor device. The reducing process is typically performed in achemical vapor deposition chamber.

The above as well as additional objectives, features, and advantages ofthe present invention will become apparent in the following detailedwritten description.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asa preferred mode of use, further objectives and advantages thereof, willbest be understood by reference to the following detailed description ofan illustrative embodiment when read in conjunction with theaccompanying drawings, wherein:

FIGS. 1A-1C depicts a process for depositing a selected compound on asubstrate in accordance with a preferred embodiment of the presentinvention; and

FIGS. 2A-2F is an example of a process for forming a tungsten plug usingtungsten nucleation from WF₆ with titanium as the reducing agent inaccordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION

The present invention provides a process for creating semiconductordevices in which a sacrificial layer of titanium on a substrate isreduced by a fluorine containing compound (RF_(x)), which contains aselected compound (R) for deposition on the substrate. The generalequation for this reaction is:

    RFx(g)+Ti(s)→R(s)+TiF.sub.4 (g) (not balanced)

With reference now to the figures, and in particular with reference toFIGS. 1A-1C, a process for depositing a selected compound on a substrateis depicted in accordance with a preferred embodiment of the presentinvention. In FIG. 1A, a titanium layer 100 is deposited on a substrate102. Ti is typically deposited using a magnetron sputter depositionsystem employing argon gas in a high vacuum chamber (typical basepressures are <1e-7 Torr). Ti can also be formed using CVD of anappropriate Ti containing precursor (for example, TiCl₄). Substrate 102is not limited to merely a silicon substrate, but may be, for example,an oxide or nitride substrate in accordance with a preferred embodimentof the present invention. In FIG. 1B, titanium layer 100 is exposed toRF_(x) through chemical vapor deposition. RF_(x) reduces the titanium intitanium layer 100. In place of titanium layer 100 is a layer 104 ofcompound R, as illustrated in FIG. 1C. R may take the form of variouscompounds, such as, for example, silicon or tungsten in accordance witha preferred embodiment of the present invention. With silicon being theselected compound for deposition, the fluorine containing compound wouldbe SiF₄ and with tungsten, the fluorine containing compound would beWF₆. Some other compounds that exist in a gas are:

AsF₅ --Arsenic penta-F to get arsenic metal

GeF₄ --Germaine to get poly-germanium (semiconductor)

SiF₄ --Silane to get poly-silicon (semiconductor)

SeF₆ --Selenium hexa-F to get selenium metal

TeF₆ --Telerium hexa-F to get telerium metal

WF₆ --Tungsten hexa-F to get tungsten metal

In particular, the restriction to the type of R group is that the Rgroup must be one that will adhere to substrate 102. Additionally,titanium also must adhere to the selected substrate 102.

Turning now to FIGS. 2A-2F, an example of a process for forming atungsten plug using tungsten nucleation from WF₆ with titanium as thereducing agent is depicted in accordance with a preferred embodiment ofthe present invention. Chemical vapor deposition is employed in thedepicted example in which the application of energy by various forms,such as heat, plasma, or radiation breaks chemical bonds and sourcematerials to associate into free radicals, which in some cases form newcompounds and in others a component of the source material deposits out.The component or new compound molecules (WF₆) move around on thesubstrate until they arrive at locations favorable for nucleation.Excess energy is given up and film growth proceeds. The reactionsinvolve pyrolysis in which a compound disassociates with the applicationof heat and then reduction in which a component of the compound is freedby reacting with another component to form a new compound, with a lowerenergy. More information on chemical vapor deposition may be found inAnner, Planar Processing Primer, Van Nostrand Reinhold, New York, N.Y.,(1990) and Wolfe and Tauber, Silicon Processing for the VLSI Era, Volume1: Process Technology, Lattis Press, Sunset Beach, Calif., (1986).

In FIG. 2A, substrate 200 is aluminum or silicon on which an SiO₂insulating layer resides with a hole 202 etched into the SiO₂ insulatinglayer. Additionally, a titanium layer 204 has been deposited bysputtering to a thickness from about 50 Å to about 5000 Å. Next, in FIG.2B, a titanium nitride (TiN) layer 206 is deposited in a layer fromabout 50 Å to about 5000 Å thick. Then, a sacrificial titanium layer 208is deposited, as depicted in FIG. 2C. In the depicted example,sacrificial titanium layer 208 is about 200 Å thick. This layer may varyfrom about 25 Å to about 200 Å in forming tungsten plugs in vias andcontacts having a hole diameter of less than 0.8 μm. Sacrificialtitanium layer 208 is employed to promote rapid nucleation of tungsten.Then, in FIG. 2D, sacrificial titanium layer 208 is exposed to WF₆ in aCVD reactor, such as a Novelus Systems Concept One Cold-Wall CVD reactoravailable from Novelus Systems, located in San Jose, Calif. WF₆ isflowed through the chamber mixed with argon for reaction with thesacrificial titanium layer 208. The sacrificial titanium layer 208 isreduced and tungsten layer 210 is formed as illustrated in FIG. 2E. Inparticular, the tungsten components of WF₆ is freed by reacting with thetitanium in sacrificial titanium layer. The compound formation duringWF₆ exposures can be expressed by the following reaction:

    2WF.sub.6 (g)+4Ti(s)→2W(s)+4TiF.sub.3 (s)

When a large fraction of sacrificial titanium layer 208 is converted toTiF₃, further W₆ F exposure causes ₄ TiF evolution by fluorination ofTiF₃ in the reaction shown below:

    WF.sub.6 (g)+6TiF.sub.3 (s)→W(s)+6TiF.sub.4 (g)

Tungsten deposition on titanium occurs by the reduction of WF₆ moleculeson the surface of sacrificial titanium layer 208. At higher pressures orlonger exposure times, titanium reduction of WF₆ can be represented bythe following reaction:

    2WF.sub.6 (g)+3Ti(s)→2W(s)+3TiF.sub.4 (g)

Thereafter, tungsten plug 212 is formed by further chemical vapordeposition of tungsten using hydrogen gas (H₂) as the reducing agent.The use of titanium as a reducing agent for WF₆ allows for a significantincrease in the nucleation of tungsten. Additionally, nucleated tungstenforms a more effective barrier to prevent diffusion of fluorine into theunderlying titanium nitride and titanium layers. Thus, titanium nitridelayer 206 and titanium layer 204 are left relatively fluorine free andthe likelihood of volcano formation is significantly reduced in such aprocess.

EXAMPLE

In creating a tungsten plug, a cluster high vacuum physical vapordeposition (PVD) tool may be used in which titanium would be sputteredat a pressure of less than 2 milli torr using argon gas in the vacuumchamber. The titanium layer would be sputtered to a thickness of 600 Åfor contacts and 300 Å for vias on a semiconductor device. Then titaniumnitride would be sputtered to form a layer that is 700 Å for contactsand 1000 Å for vias. Then a sacrificial titanium layer would besputtered to form a layer of titanium that is 200 Å thick. Next, thewafer is then placed in a CVD chamber and heated to 445° C. The chamberis then held at 40 torr in a mixture of argon and hydrogen gas. Thenucleation step in which the sacrificial titanium layer would be reducedwould take place in a mixture of argon, hydrogen gas, and WF₆ for 10 orless seconds. The remainder of the nucleation takes place in a mixtureof argon, hydrogen, and WF₆ from about 15 to about 40 seconds dependingon the final tungsten thickness required. The gas ratio used in the twonucleation steps are Ar:H₂ :WF₆ =30:30:1.

Thus, the present invention provides an improved process for depositingselected compounds on a substrate. With respect to deposition oftungsten, the present invention provides faster tungsten nucleation anda higher deposition rate for the overall tungsten film, which results inhigher throughput for the process. Additionally, lower fluorine contentis found in the underlying titanium nitride and the titanium layers.Another benefit of the present invention with respect to deposition oftungsten is smaller tungsten grain size and reduced likelihood offorming volcanos. As a result, more conductive titanium nitride/titaniumlayers are formed lowering contact and via resistances. This processalso may be employed for insulating film deposition on surfaces.

The description of the preferred embodiment of the present invention hasbeen presented for purposes of illustration and description, but is notintended to be exhaustive or limit the invention in the form disclosed.Many modifications and variations will be apparent to those of ordinaryskill in the art. For example, although the present invention has beendescribed with reference to a semiconductor process, the method of thepresent invention also may be applied to other types of processes inother technologies, such as, for example, the surface of fan blades foraircraft or tungsten coatings for tool bits where reduction according tothe present invention is employed. The embodiment was chosen anddescribed in order to best explain the principles of the invention andthe practical application to enable others of ordinary skill in the artto understand the invention for various embodiments with variousmodifications as are suited to the particular use contemplated.

What is claimed is:
 1. A method in a semiconductor process for forming alayer of a selected reducing agent on a substrate of a semiconductordevice:forming a layer of titanium on the substrate; and reducing thelayer of titanium using a gaseous form of a fluorine containingcompound, the fluorine containing compound including the selectedreducing agent, wherein a layer of the selected reducing agent is formedon the substrate of the semiconductor device.
 2. A method in asemiconductor process for forming a layer of a selected reducing agenton a substrate of a semiconductor device:forming a layer of titanium onthe substrate; and reducing the layer of titanium using a gaseous formof a fluorine containing compound, the fluorine containing compoundincluding the selected reducing agent, wherein a layer of the selectedreducing agent is formed on the substrate of the semiconductor devicewherein the substrate is silicon, wherein the step of forming a layer oftitanium on the substrate comprises;forming a first layer of titanium onthe substrate; forming a titanium nitride layer in contact with thefirst titanium layer; and forming a second titanium layer in contactwith the titanium nitride layer, wherein the second titanium layer isthe layer of titanium formed.
 3. A method in a semiconductor process forforming a layer of a selected reducing agent on a substrate of asemiconductor device:forming a layer of titanium on the substrate; andreducing the layer of titanium using a gaseous form of a fluorinecontaining compound, the fluorine containing compound including theselected reducing agent, wherein a layer of the selected compound isformed on the substrate of the semiconductor device wherein thesubstrate is oxide and the fluorine containing compound is SiF₄ and theselected reducing agent is Si.
 4. The method of claim 1, wherein thefluorine containing compound is selected from the group consisting ofAsF₅, GeF₄, SeF₆, and TeF₆.
 5. The method of claim 1, wherein thesubstrate is selected from the group consisting of silicon and silicondioxide.
 6. The method of claim 1, wherein the layer of titanium is fromabout 25 Å to about 200 Å thick.
 7. The method of claim 1, wherein thereducing step is performed in a chemical vapor deposition reactor.
 8. Amethod of forming a semiconductor device including a tungsten plugcomprising:forming a cavity in a substrate wherein the cavity has abottom and sidewalls; forming a first layer of titanium on the bottomand sidewalls of the cavity; forming a layer of titanium nitride incontact with the first layer of titanium; forming a second layer oftitanium in contact with the layer of titanium nitride; and reducing thesecond layer of titanium with WF₆ in a gaseous form, wherein a layer oftungsten is formed.
 9. The method of claim 8, wherein the second layerof titanium is substantially reduced by the WF₆.
 10. The method of claim8 further comprising filling the cavity with tungsten after reducing thesecond layer of titanium.
 11. The method of claim 8, wherein the secondlayer of titanium is about 200 Å thick.
 12. The method of claim 8,wherein a first portion of the substrate is silicon and a second portionof the substrate is oxide.
 13. The method of claim 8, wherein the secondlayer of titanium has a thickness from about 25 Å to about 200 Å. 14.The method of claim 8, wherein the layer of titanium nitride has athickness from about 50 Å to about 5000 Å.
 15. The method of claim 8,wherein the first titanium layer has a thickness from about 50 Å toabout 5000 Å.
 16. The method of claim 8, wherein the step of reducingthe second layer is performed in a chemical vapor deposition reactor.17. A method in a semiconductor process for forming a layer of siliconon an insulator comprising the steps of:forming a layer of titanium onthe insulator; and reducing the layer of titanium using SiF₄ in agaseous form, wherein a layer of polycrystalline or armorphous siliconis formed on the insulator.
 18. The method of claim 17, wherein theinsulator is oxide.
 19. The method of claim 17, wherein the layer oftitanium is from about 25 Å to about 200 Å thick.
 20. The method ofclaim 17, wherein the step of forming a layer of titanium on theinsulator comprises sputtering titanium onto the insulator.
 21. Themethod of claim 12, wherein the insulator comprises silicon dioxide. 22.The method of claim 8, wherein the step of reducing the second layer oftitanium comprises reducing the second layer of titanium with WF₆ in agaseous form and a chemical vapor deposition chamber having a pressureof about 40 torr in a mixture of argon and hydrogen gas.
 23. The methodof claim 22, wherein the argon gas, hydrogen gas, and WF₆ in gaseousform have a ratio of 30:30:1.
 24. The method of claim 22, wherein thestep of reducing the second layer of titanium with WF₆ in a gaseous formcomprises reducing the second layer of titanium with WF₆ in a gaseousform in a chemical vapor deposition chamber heated to about 445° C. 25.A method in a semiconductor process for forming a layer of a selectedreducing agent on a semiconductor device comprising:forming a layer of anucleating compound, wherein the nucleating compound forms a fluorinegas biproduct in response to exposure to a fluorine containing compoundin a gaseous form, wherein the fluorine containing compound includes theselected reducing agent; and exposing the layer to the fluorinecontaining compound in a gaseous form, wherein nucleation of thenucleating compound in the layer occurs forming the layer of theselected reducing agent, wherein the layer of the selected reducingagent is formed on the semiconductor device, replacing the layer of thenucleating compound.
 26. The method of claim 25, wherein the layer ofnucleating compound is a layer of titanium.
 27. The method of claim 26,wherein the fluorine containing compound in gaseous form is WF₆.
 28. Amethod in a semiconductor process for forming a layer of a selectedreducing agent on a semiconductor device comprising:forming a layer of anucleating compound, wherein the nucleating compound forms a fluorinegas biproduct in response to exposure to a fluorine containing compoundin a gaseous form, wherein the fluorine containing compound includes theselected reducing agent; and exposing the layer to the fluorinecontaining compound in a gaseous form, wherein nucleation of thenucleating compound in the layer occurs forming the layer of theselected reducing agent, wherein the layer of nucleating compound is alayer of titanium and wherein the fluorine containing compound ingaseous form is SiF₄ and the selected reducing agent is Si.
 29. Themethod of claim 26, wherein the fluorine containing compound is selectedfrom the group consisting of AsF₅, GeF₄, SeF₆, and TeF₆.